feat: Add nested_column_iter_to_arrays to deserialize inner columns

src/io/parquet/read/deserialize/mod.rs

@@ -214,3 +214,22 @@ where
             .map(|x| x.map(|x| x.1)),
     ))
 }
+
+/// Basically the same as `column_iter_to_arrays`, with the addition of the `init` parameter
+/// to read the inner columns of the nested type directly, instead of reading the entire nested type.
+pub fn nested_column_iter_to_arrays<'a, I: 'a>(
+    columns: Vec<I>,
+    types: Vec<&PrimitiveType>,
+    field: Field,
+    init: Vec<InitNested>,
+    chunk_size: Option<usize>,
+    num_rows: usize,
+) -> Result<ArrayIter<'a>>
+where
+    I: Pages,
+{
+    Ok(Box::new(
+        nested::columns_to_iter_recursive(columns, types, field, init, num_rows, chunk_size)?
+            .map(|x| x.map(|x| x.1)),
+    ))
+}

src/io/parquet/read/mod.rs

@@ -37,7 +37,7 @@ use crate::{array::Array, error::Result};
 use crate::types::{i256, NativeType};
 pub use deserialize::{
     column_iter_to_arrays, create_list, create_map, get_page_iterator, init_nested, n_columns,
-    InitNested, NestedArrayIter, NestedState, StructIterator,
+    nested_column_iter_to_arrays, InitNested, NestedArrayIter, NestedState, StructIterator,
 };
 pub use file::{FileReader, RowGroupReader};
 pub use row_group::*;

tests/it/io/parquet/deserialize.rs

@@ -0,0 +1,87 @@
+use std::fs::File;
+
+use arrow2::{
+    array::StructArray,
+    datatypes::DataType,
+    error::{Error, Result},
+    io::parquet::read::{
+        infer_schema, n_columns, nested_column_iter_to_arrays, read_columns, read_metadata,
+        to_deserializer, BasicDecompressor, InitNested, PageReader,
+    },
+    util::test_util::parquet_test_data,
+};
+
+#[test]
+fn test_deserialize_nested_column() -> Result<()> {
+    let testdata = parquet_test_data();
+    let path = format!("{testdata}/nested_structs.rust.parquet");
+    let mut reader = File::open(&path).unwrap();
+
+    let metadata = read_metadata(&mut reader)?;
+    let schema = infer_schema(&metadata)?;
+
+    let num_rows = metadata.num_rows;
+    let row_group = metadata.row_groups[0].clone();
+
+    let field_columns = schema
+        .fields
+        .iter()
+        .map(|field| read_columns(&mut reader, row_group.columns(), &field.name))
+        .collect::<Result<Vec<_>, Error>>()?;
+
+    let fields = schema.fields.clone();
+    for (mut columns, field) in field_columns.into_iter().zip(fields.iter()) {
+        if let DataType::Struct(inner_fields) = &field.data_type {
+            let mut array_iter =
+                to_deserializer(columns.clone(), field.clone(), num_rows, None, None)?;
+            let array = array_iter.next().transpose()?.unwrap();
+            let expected_array = array
+                .as_any()
+                .downcast_ref::<StructArray>()
+                .unwrap()
+                .clone();
+
+            // deserialize inner values of struct fields.
+            let init = vec![InitNested::Struct(field.is_nullable)];
+            let mut values = Vec::with_capacity(inner_fields.len());
+            for inner_field in inner_fields {
+                let n = n_columns(&inner_field.data_type);
+                let inner_columns: Vec<_> = columns.drain(0..n).collect();
+
+                let (nestd_columns, types): (Vec<_>, Vec<_>) = inner_columns
+                    .into_iter()
+                    .map(|(column_meta, chunk)| {
+                        let len = chunk.len();
+                        let pages = PageReader::new(
+                            std::io::Cursor::new(chunk),
+                            column_meta,
+                            std::sync::Arc::new(|_, _| true),
+                            vec![],
+                            len * 2 + 1024,
+                        );
+                        (
+                            BasicDecompressor::new(pages, vec![]),
+                            &column_meta.descriptor().descriptor.primitive_type,
+                        )
+                    })
+                    .unzip();
+
+                let mut inner_array_iter = nested_column_iter_to_arrays(
+                    nestd_columns,
+                    types,
+                    inner_field.clone(),
+                    init.clone(),
+                    None,
+                    num_rows,
+                )?;
+                let inner_array = inner_array_iter.next().transpose()?;
+                values.push(inner_array.unwrap());
+            }
+            let struct_array = StructArray::try_new(field.data_type.clone(), values, None)?;
+
+            assert_eq!(expected_array, struct_array);
+        }
+    }
+
+    Ok(())
+}

tests/it/io/parquet/mod.rs

@@ -14,6 +14,7 @@ use arrow2::{
     types::{days_ms, NativeType},
 };
 
+mod deserialize;
 #[cfg(feature = "io_json_integration")]
 mod integration;
 mod read;